In water sports, a typical tow boat is operated at speeds in excess of 15 mph. At these speeds, the boat hull creates a trailing wake with a low, turbulent middle developing near the stern of the boat. The outer edge of the wake forms a V-shaped lip with water outside the middle of the wake.
As a boat travels through the water, the stern of the boat hull lowers in the water and the bow rises at an angle of attack to oncoming water. The stern of the boat hull displaces water, which effectively creates the wake behind the boat. A heavier boat lying lower in the water will create a larger wake due to the greater water displacement. The shape of the hull also affects the wake shape and size.
While often referred to generically as “waterskiing” and “wake boarding”, water sports utilizing a tow boat generally encompass many different sports using skis, skates, boards, water foils, and even bare feet. With the advent of wakeboarding and trick skiing, the wake shape has become a more prominent component of the water-ski sports and tow boats. Many of these sports, especially wakeboarding, primarily focus on use of the wake to perform aerial maneuvers.
Typical wakeboards have a wide and flat form for sliding over the water surface. In comparison to water skiing or hydrofoil water sports, a wakeboarder is pulled through the water at a lower speed, typically in the range of 15–25 mph.
Wakeboarding and trick skiing enthusiasts, in particular, use the wake repetitively as a “ramp” by jumping over the wake. As such, the size of the wake is of primary concern for wakeboarders. As the sport has matured, wakeboard enthusiasts have developed increasingly complex tricks like spins, grabs, and flips. A larger wake allows wakeboarders to get more “air” when crossing over the wake, meaning that the rider can jump higher over the wake.
In order to increase the wake size, enthusiasts and boat designers have employed various techniques for lowering the stern of the boat and increasing wake size. One simple method is to place weights in the stern of the boat. Some tow boats have been designed specifically with additional weight in the transom for this purpose. The additional weight lowers the stern of the boat during towing and increases the size of the wake, however, there are several disadvantages to this method. The lowered floating position of the boat translates into slower cruising speeds. This is disadvantageous when the boat is used for cruising rather than skier towing. It can also be disadvantageous when one desires to use the same boat for pulling wakeboarders at slower speeds with a larger wake and skiers at high speeds with a small wake. One cannot increase or decrease the wake size and maintain a top cruising speed without stopping ashore to load or unload the weights.
Furthermore, the additional weight poses a safety risk. Every boat has a maximum load capacity, and the additional weight lowers the margin of safety. Typical weight additions can be from 800–1200 pounds. In a smaller boat, this additional weight combined to the passenger weight can be close to the maximum rated load-capacity.
Another class of boats has been developed to adjust the wake. These tow boats, known generally as “tournament boats,” have been developed for competitive skiing. The hull section of these boats are relatively flat such that minimal wake is produced at speed. The smaller wake is suitable for high-speed skiing and slalom competitions requiring precise turning patterns. Because these boats do not employ a deep V-shaped hull, they have lower top speeds typically under 50 mph. These boats are said to run “wet,” meaning that they have high drag from the added area contacting the water surface. Tournament boats also employ control devices for changing the size of the wake. Several devices have been employed to balance the need for less drag with a smoother wake. An example of such a control device is disclosed in U.S. Pat. No. 5,549,071. Generally, these devices and designs minimize the wake size, which is not desirable for wakeboarding and trick skiing.
In contrast to tournament boats with smooth wakes, some boats have a hull designed with a deep deadrise. A deep deadrise creates a larger wake preferable for trick skiing. However, all the hull designs require a tradeoffs between speed and wake size.
U.S. Pat. No. 4,915,048 to Stanford discloses a submerged hydrofoil device for adjusting the trim and stability of a tow boat. The hydrofoil is also integral to the boat design and cannot be retracted for cruising. Norwegian patent no. 86,945 to Troenge is directed to a retractable stern-mounted foil. However, similar to the Stanford patent, Troenge is directed to improved stability and not adjustment of the wake size. Both patents are directed to adjusting and balancing the pitch and yaw forces and angle of attack of the tow boat.
Some submerged hydrofoil devices have been employed specifically to increase wake size. These hydrofoil devices increase downforce on the boat hull and in turn increase the wake size. Fixed hydrofoil devices are limited in that they do not allow for adjustment of the wake size; the single position of the hydrofoil allows for only a single wake size at a particular speed. In particular, known hydrofoil devices either require manual removal of the hydrofoil at cruising speed or they cannot be removed at all. If removable, these devices require manually removing the hydrofoil. If the hydrofoil is not retracted, it decreases the top speed of the boat. Thus, known hydrofoil devices are fixed or cannot be easily adjusted between cruising and towing conditions.
Newer methods have been devised which allow for increasing the wake size at tow speed without sacrificing cruising speed. One such method uses of an inflatable bladder. When towing riders, the bladder can be filled with water to increase weight in the stern of the boat. At cruising speeds, the water in the bladder can be excreted out of the boat. This device has the limitation of requiring users to wait for the bladder to be filled and emptied and also has limited adjustability. Moreover, the increased weight of the bladder also lowers the loading margin of safety as discussed.
What is needed is a method and apparatus that allows adjustable wake modification while overcoming the above-mentioned disadvantages.